The typical round baler includes a main frame adapted to advance over a field, as by being drawn by a tractor. Such baler will employ a belt and roll system arranged so that portions or stretches of the belt operate among several rolls to define a bale-forming chamber in which the cylindrical bale is formed as opposed stretches of the belt move in opposite linear directions. Provision must be made for enabling certain of the rolls to change position so as to supply belt portions to the ever-increasing bale-forming chambers and such provision should include means for maintaining proper bale tension and thus bale density as well as for taking up slack in the belt when the completed bale is discharged, as via a rear-dump gate. It has been heretofore found difficult to control the expansion of the bale chamber and resort has often been had to various kinds of guides and shields in addition to the rolls. According to the present invention, bale chamber control, along with the maintenance of proper belt tension and bale density, is achieved in a simple manner by the provision of a pair of intermediate rolls disposed in such manner as to substantially close the top of the bale chamber and to confine bale formation below these rolls and the lower rolls at the inlet to the chamber. Control means is arranged to act on these rolls in such manner as to attain proper density without unduly increasing power consumption. The control means includes a combination of a hydraulic system and a springflexible tension means system, with provision for adjustment. The hydraulic system is tied in with the hydraulic means that operates the bale discharge gate. Further features will appear as the disclosure progresses in connection with a preferred embodiment of the invention.